The Association of Cinnabar and Bitumen in Mercury Deposits of the California Coast Ranges

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Abstract

A review of the literature from around the world indicates that the association of bitumen and cinnabar in mercury deposits is widespread. Such deposits are commonly associated with sedimentary rocks; the compositions of igneous rocks in the vicinity of these deposits are highly variable, suggesting that anomolous heat rather than igneous rock type may be important to their formation. These deposits are commonly spatially coincident with anticlines or domes, which suggests that the focusing of buoyant fluids may be critical to their formation. The morphology of mercury ore bodies throughout the world is controlled by permeability, whether primary or secondary. On the deposit scale, mercury ore is spatially correlated with solid or liquid bitumens, or gas.

Within the USA, cinnabar-bitumen ore deposits are most prevalent in the California Coast Ranges. The variability of physical occurrence and chemical composition of bitumen is illustrated by samples of ore and gangue from nearly a dozen currently defunct mercury deposits. Common modes of occurrence for bitumen include: (a) clots and masses along the centers of silica and/or carbonate veins, (b) thin films along vein margins and/or crystal growth fronts, (c) masses within vugs of breccia veins, particularly along the hanging walls, (d) discrete microscopic fluid inclusions, (e) droplets within so-called “froth veins” of silica and/or carbonate, and (f) fine disseminations within siliceous sinter. At the hand-sample and microscopic scales, bitumen and cinnabar are spatially correlated. Bitumens are generally low in saturates and contain significant amounts of aromatics, NSOs, and asphaltenes. Some of the most abundant compounds have been identified as methylphenanthrenes, dimethylphenanthrenes, chrysene, methylchrysenes, benzofluoranthene(s), and benzopyrene(s). The range in carbon number of bitumens is typically small, possibly reflecting a process of natural fractionation arising from involvement in hydrothermal systems.

The chemical compositions of these bitumen samples strongly reflect the influence of thermal alteration. Results of recent shale retort experiments have shown that trace amounts of mercury in sedimentary rocks can be liberated to a gas phase in response to heating (Olsen et al. 1985). It is likely that the combination of high geothermal gradients, high fluid fluxes, and focusing of fluids all may have contributed to the generation, migration, and concentration of bitumen along with mercury in these deposits.